Environmental issues of nitrogen-containing fertilisers

Question 1

What happens in natural ecosystems?

Answer 1

• in natural ecosystems minerals such as nitrate ions, which are removed from the soil by plants are mainly returned when the plant is decomposed
• however in artificial systems the crop is removed and so the nitrate is not returned an has to be replaced
• this is done by the addition of natural or artificial fertilisers

Question 2

What are the effects of nitrogen-containing fertilisers?

Answer 2

• nitrogen is an essential component of biological molecules such as proteins and is needed for growth and therefore an increase in the area of leaves
• this increases the rate of photosynthesis and improved crop productivity
• nitrogen-containing fertilisers have benefited us providing us with cheaper food

Question 3

What are the detrimental effects of the use of nitrogen-containing fertilisers?

Answer 3

• reduces species diversity
• leaching
• eutrophication

Question 4

How do nitrogen-containing fertilisers reduce species diversity?

Answer 4

• because nitrogen-rich soils favour the growth of gasses, nettles and other rapidly growing species
• these out-compete many other species, which die as a result
• species-rich hay meadows, only survive when soil nitrogen concentrations are low enough to allow other species to compete with these gases

Question 5

What causes eutrophication?

Answer 5

-caused by leaching of fertiliser into watercourses

Question 6

What is leaching?

Answer 6

• leaching is the process by which nutrients are removed from the soil
• rainwater will dissolve any soluble nutrients, such as nitrate ions and carry them deep into the soil, eventually beyond the reach of plant roots
• the leached nitrate ions find their way into watercourses, such as streams ands rivers, that in turn may drain into freshwater lakes
• here they may have a harmful effect on humans if the river or lake is a source of drinking water
• very high nitrate ion concentrations in drinking water can prevent efficient oxygen transport in babies and a link to stomach cancer in humans has been suggested
• the leached nitrate ions are also harmful to the environment as they can cause eutrophication

Question 7

What is the process of eutrophication?

Answer 7

• in most lakes and rivers there is naturally very low concentration of nitrate and so nitrate ions are a limiting factor for plant and algae growth
• as the nitrate ion concentration increases as a result of leaching, it ceases to be a limitation factor for the growth of plants and algae whose populations both grow
• as algae mostly grow at the surface, the upper layers of water become densely populated with algae, this is called an algal bloom
• this dense surface layer of algae absorbs light and prevents it from penetrating to lower depths
• light then becomes the limiting factor for the growth of plants and algae at lower depths and so they eventually die
• the lack of dead plants and algae is no longer a limiting factor for the growth of saprobiontic bacteria and so these populations too grow, using the dead organisms as food
• the saprobiontic bacteria require oxygen for their respiration creating an increased demand for oxygen
• the concentration of oxygen in the water is reduced and nitrates are released from the decaying organisms
• oxygen then becomes the limiting factor for the population of aerobic organisms, such as fish, these organisms ultimately die as the oxygen is used up altogether
• without the aerobic organisms, there is less competition for the anaerobic organisms whose populations now rise
• the anaerobic organisms further decompose dead material, releasing more nitrates and some toxic wastes, such as hydrogen sulphide, which make the water putrid

Question 8

Which factors contribute to eutrophication?

Answer 8

• organic manure
• animal slurry
• human sewage
• ploughing old grassland
• leaching of artificial fertilisers is the main cause

Question 9

What are troubled waters?

Answer 9

• a farmer applied a large quantity of fertilisers toil fields next to a small lake
• a period of heavy rain followed
• after 10 days, scientists monitoring the lake noticed changes to the algal population, the clarity of the water and the levels of dissolved oxygen

Question 10

How do you measure the clarity of water?

Answer 10

-measurements are taken by lowering a black-and-white disc called the secchi disk into the water and recording the depth at which it is no longer visible